Laminar thermoplastic film constructions

ABSTRACT

Laminar thermoplastic film constructions comprising a layer of low density polyethylene bonded to a layer of a dissimilar polymer blend comprising high density polyethylene and polyethylene copolymers. In particular such copolymers comprise polyethylene copolymerized with another alpha olefin containing from about 3 up to about 15 carbon atoms. Such copolymers are also characterized by being linear, low density polymers having densities which are below about 0.94 grams/cc.

This is a continuation of application Ser. No. 126,932, filed Mar. 3,1980, now U.S. Pat. No. 4,303,710 issued Dec. 1, 1981 which is acontinuation of application Ser. No. 934,235 filed Aug. 16, 1978, nowabandoned.

DESCRIPTION OF THE PRIOR ART

Thermoplastic bags, and in particular polyethylene bags, have in recentyears gained prominence in the packaging of a wide variety of goods suchas dry goods, comestibles and the like. Most recently, polyethylene bagshave emerged as the preferred packaging material for refuse materialsand, in fact, many communities across the country have mandated thatrefuse be packaged and contained in such a manner. The advantagesoffered are obvious and include a hygenic means for the containment ofgarbage and waste materials; the bag provides some protection of thecontents from insects, ruminants and other animals which would normallybe attracted by the bag contents. Such bags are conventionly employed asdisposable liners for trash cans whereby when the trash containers havebeen filled to capacity, the bag mouth is gathered and twisted closedand raised out of the container, leaving the interior of the containerfree from contamination and ready to receive another bag liner. Thetwisted bag mouth may be secured in a conventional manner employingwire-twistems or similar fasteners and subsequently the closed, loadedbag is disposed of. Alternatively, such bags may be employed in anunsupported condition as receptables. Prior art polyethylene branchedlow density homopolymer bags however lack stiffness and when articlesare loaded into such bags difficulties are encountered in keeping thebag mouth open, requiring excessive digital manipulation.

Another of the most common drawbacks in the employment of polyethylenebags in waste disposal is their tendency to rupture under load stressesand, also, their fairly low puncture resistance. When a loaded bag ispunctured, by an internal or external element, it is characteristic ofthe polyethylene film to zipper, i.e., the puncture tear rapidlypropagates across or down the bag wall.

Numerous attempts have been made in the past to remedy the aforenoteddeficiencies, the most obvious being to increase the film gauge, i.e.,make the bag walls thicker and therefore stronger. However, substantialgauge increases are necessary to achieve substantial bag strengthening,on the order of 50% to 150%, and the product costs are increased indirect proportion to the increased amount of resin employed in each bag.Attempts to replace the relatively low cost polyethylene with otherresins which exhibit improved strength characteristics have been largelyunsuccessful for reasons including the unfavorable economics associatedwith the more costly resin substitutes.

SUMMARY OF THE INVENTION

In accordance with the present invention it has been found thatthermoplastic film structures which contain a predominant amount ofrelatively low cost resinous materials commonly used in the prior artfabrication of bags such as, for example, general purpose, low densitypolyethylene branched homopolymer resin may be fabricated into articlessuch as bags which have improved stiffness, i.e., modulus, and strengthcharacteristics over prior art polyethylene bags. In general it has beenfound that a multi-layer structure comprising at least one layer of lowdensity, general purpose polyethylene resin having a thickness on theorder of from about 50% to 90% and preferably from about 65% up to about85% of the overall laminate thickness may be bonded to a second layer,the second layer contributing the balance of the overall multi-layerthickness of a blend of resins. For example, the second layer may beconstituted by a relatively thin layer of a resinous blend whichcomprises a high density polyethylene homopolymer resin and a linear lowdensity polyethylene copolymer, which may be a copolymer of ethylene andanother alpha olefin having from about three up to fifteen carbon atomsand a density of below about 0.94 grams per c.c. The preferredalpha-olefin comonomers comprise at least one C₄ to C₈ olefin. Minoramounts of a colorant masterbatch material, on the order of less thanabout 5% by weight, such as a blend of low density polyethylene and aninorganic pigment may also be used. It has been found that whenstructures such as bags are fabricated from such laminar film materials,the branched low density polyethylene layer preferably constituting theinterior bag surface, such bag structures offer improved strengthcharacteristics as contrasted to the aforedescribed prior artpolyethylene bag structures. Additionally, such strength characteristicsare not achieved by sacrificing material economics as hereinabovediscussed since the laminar bag structure of the present inventioncontains a predominant amount, i.e., up to about 85% of the overalllaminar thickness, of low cost general purpose branched low densitypolyethylene resin.

BRIEF DESCRIPTION OF DRAWINGS

The FIGURE is a schematic side elevation, in cross section, of one formof extrusion apparatus which may be employed for the production of thelaminar films of the present invention, with certain segments enlargedfor clarity.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Numerous techniques have been described in the prior art for theformation of multilayer laminar thermoplastic film constructionsincluding preforming a first film and subsequently melt extrudinganother film onto its surface whereby a two layer laminate is formed.Other techniques which have been developed in more recent years includea technique which is referred to as coextrusion, a process wherebymolten or semi-molten layers of different polymer melts are brought intocontact and subsequently cooled. Examples of such coextrusion techniquesare described in U.S. Pat. Nos. 3,508,944 and 3,423,010. Although any ofthe aforedescribed techniques may be suitable in formation of thelaminar structures of the present invention a particularly preferredtechnique is to produce the present laminates by extrusion of separatepolymer melts from tubular die orifices which are concentric causing theseparate molten or semi-molten streams to be extruded coaxially and thenmerged together outside of the die orifices whereby upon subsequentcooling a tubular laminate is produced.

In producing the multi-layer film of the present invention, intended forbag structures in one particular application, it has been found thatcertain particularly desirable physical characteristics should beexhibited by the individual lamina. For example in bag constructions theouter layer, which may comprise from about 10% up to 50% of the overalllaminate thickness, must be preferably stiff, i.e., have a relativelyhigh tensile modulus; it must be tough, i.e., resistant to impactforces; it should exhibit good elongation under stress; and, finally,have a high degree of tear resistance particularly in the transversedirection of the layer, i.e., the direction which is transverse to theextrusion direction. The physical characteristics which are particularlydesirable in the thicker interior laminar bag layer include ease of heatsealing over wide ranges of temperature and pressure; and a high degreeof tear resistance particularly in the layer's machine direction.

The degree of orientation in each of the respective laminar layers is animportant factor with respect to the overall physical properties of themulti-layer structure. It has been found that two types of orientationof the polymer crystallites occur in blown film extrusion by the trappedair method. The first type occurs by flow through the die lips and thisorientation tends to align the crystallites formed upon cooling in thedirection of flow (MD). In a linear polymer with long, straight chains,the crystallites are oriented in the machine direction. With morebranching of the chain, as in ordinary low density branched polyethylenethe crystallites tend to be in a somewhat more random orientation. Theorientation of high density polyethylene, since it is linear and morecrystalline, thus is quite strong compared to branched low densitypolyethylene. From this die effect alone, the net result is a highlyoriented film in the machine direction (MD) with little transversedirection (TD) orientation. In the homopolymer progression from ordinarylow density polyethylene to high density polyethylene, as the densityincreases and polymer branching decreases, the material is more subjectto orientation. High density polyethylene is highly oriented and thussusceptability to tearing in the machine direction (MD) is very high.

It has been found that the second type of orientation in the blown filmprocess is the blow-up ratio (BUR) effect. Since this stretching of thefilm expands the bubble to larger diameters, the stress on polymercrystallites is multi-directional in nature and thus helps counteractthe MD orientation associated with the die effects. As BUR increases, TDorientation effects increase at some drop in MD properties. Improvedtear resistance thus can be achieved in the normally weak TD direction.

Low density polyethylene normally is run in the range of 1.5-3.0:1blow-up ration (circumference of the bubble:circumference of the annuladie) in an attempt to balance the properties between machine direction(MD) and transverse direction (TD). In contrast high densitypolyethylene orients strongly in the machine direction due to the dieeffect, giving very poor properties at low density polyethylene typeblow-up ratios. Economics and ease of handling the molten polymerstrongly discourage such large blow-up ratios but tear is a key propertyin the bag type product. The present invention permits film to run atlow density polyethylene rates and BUR conditions (i.e., 1.5 to 3:1ratio) with the additional stiffness and strength of the high densitypolyethylene-ethylene and α-olefin blend in the outer layer.

There is illustrated in FIG. 1 one form of extrusion apparatus which maybe employed to produce the laminar films of the present invention. Asshown two thermoplastic extruders 11 and 12 feed dissimilar moltenthermoplastic resins to common die member 13. Tubular extrusion die 13has two concentric annular passages to separately accommodate and shapethe individual resinous streams until they exit from concentric dieorifices 14 and 14'. Shortly after emerging from orifices 14 and 14' theconcentric, coaxial, molten or semi-molten tubes merge and become bondedtogether to form a two layered laminar tube 15. Air is provided (byconventional means not shown) to inflate and support tube 15 until tube15 is collapsed downstream from die 13 by conventional counter-rotatingcollapsing rollers (not shown), i.e., a conventional entrappedair-bubble tubular extrusion process. The collapsed laminar tubing issubsequently passed to a wind-up station (not shown) or on to furtherprocessing, e.g., a bag making operation.

In practice, pelletized resinous materials to be fed to the extrusionsystem illustrated in FIG. 1 is air-veyed by a vacuum unloader from asupply source and fed to separate feeder tanks which are mounted abovethe individual extruders 11 and 12 illustrated in FIG. 1. Each of theresinous components in the blend compositions which are fed to extruder11 (i.e., the extruder which supplies a molten resinous blend to die 13to form outer layer 16) are volumentrically measured and dropped into amixer located above extruder 11, the order of addition is not critical.The mixer is actuated at 120 RPM for approximately 15 seconds and thenthe premixed blend is fed to the extruder feed zone (not shown). For theprimary extruder (i.e., extruder 12 which is employed to form the innerlayer 17) a resin consisting essentially of branched, low densitypolyethylene is used as a feed material.

The primary extruder 12 which was employed in the following examplecomprised a 6 inch diameter screw which was driven by a 250 HP motor.The screw had an L/D ratio of 28:1 The extruder barrel was a standarddesign and equipped with external jackets employed for the circulationof temperature control fluids therein and/or conventional electricresistance band heating elements positioned around the barrel.

The secondary extruder 11, i.e., that extruder which feeds moltenresinous blend mixtures to die 13 to form outer layer 16 of the laminarstructure, had a41/2 inch screw diameter and an L/D ratio of 24:1. Theextruder barrel for extruder 12, was likewise equipped with externaljackets for circulation therein of temperature control fluids and/orelectrical resistance band heaters spaced along the length of the barrelto control the temperatures of the molten polymer inside the barrel.

Die 13, as shown in FIG. 1, is a coextrusion die with the primaryextruder 12 feeding material which will eventually constitute layer 17and secondary extruder 11 feeding material to die 13 which willeventually constitute outer layer 16. The annular die lips haveapproximately a 0.040 inch annular gap which form orifices 14 and 14'with a 1/2 to 2 inch length angled lip section in the die so that theindividual concentric tubes are separated as they exit from die 13 byapproximately 1/32 inch. As a result of the separation, the film layersare joined above the die as illustrated in FIG. 1 to form laminar tube15.

Upon exit from die 13 the extruded concentric tubes 16 and 17 areoriented by internal air pressure trapped within the tube between thedie 13 and the film collapsing nips (not shown) which inflates the tubeto between 2 and 2.5 times the circumference of the die orifice. This isessentially a conventional entrapped air bubble extrusion technique.

While the internally trapped air is stretching the film, a high velocityair stream supplied by air ring 18 as shown in FIG. 1, impinges in agenerally vertical direction on the extruded tube to cool the moltenpolymer. The combination of internal air expansion and high velocityimpingement of air from air ring 18 causes the layers to contract whilestill in the molten state and thereby forming a strong interfacial bondas the contacting layers cool and solidify.

Prior to passage of tube 12 to the nip rollers the formed film tube isconventionally collapsed by a frame of horizontally wooden slats locatedin an inverted V shape with the angle between the legs of the Vapproximately 30° to 35°. This V frame gradually flattens the film tubeuntil, at the apex of the V, the tube is completely collapsed by the niprollers which may consist of a rubber roll and a steel driven roller.The nip rollers function to draw the tube from the extrusion die 13 andalso effect an air seal for the entrapped air bubble in the tube.Subsequent to passing the flattened tube through the nip rollers, thefilm is either wound into rolls or passed through bag making machineryor the like to form a finished product.

As hereinabove discussed, the outer layer of the laminar film structuresof the present invention preferably comprise a blend of thermoplasticresins and in particular blends of high density polyethylene togetherwith a linear low density polyethylene-alpha olefin copolymer. Whilethese copolymers may contain alpha-olefins having 3-5 carbon atoms thepreferred copolymers include polyethylene copolymerized with anotheralpha olefin including C₄ to C₈ alpha olefins such as octene-1,butene-1, hexene-1 and 4-methylpentene-1. The preferred concentration byweight of the alpha olefin which is copolymerized with polyethylene isfrom about 2.0% up to about 10% by weight. In the following specificembodiments the linear low density copolymer of polyethylene with about4.8% by weight of octene copolymerized therewith. It has been found thatwhen such a blend comprises the exterior laminar tube layer, theresultant laminates exhibit greatly improved modulus and tearresistance.

In the following Table I there is presented a listing of pertinent resinphysical properties of the various polyolefin materials which wereemployed in the succeeding examples.

                  TABLE I                                                         ______________________________________                                                                    ASTM                                              Property          Value     Test Method                                       ______________________________________                                        Low Density Polyethylene Resin (For Inner Layer                               Polyethylene Component)                                                       Melt Index, g/10 min                                                                            2.25      D-1238-65T                                        Density, g/cc     .921      D-1505-68                                         Tensile at Yield                                                              (20"/min) · psi                                                                        1331      D-638-68                                          Tensile at Break                                                              (20"/min) · psi                                                                        1688      D-638-68                                          Elongation at Break, %                                                                          603       D-638-68                                          Elastic Modulus, psi                                                                            24635     D-638-68                                          Stiffness in Flexure,                                                         psi               800       D-747-63                                          Hardness, Shore D D44       D-2240-68                                         Vicat Softening                                                               Point, °F. 217       D-1525-65T                                        Brittleness Tempera-                                                          ture, °F.  below     D-746-64T                                                           -105                                                        Physical Properties - Linear Low Density                                      Polyethylene - Octene-1 Copolymer Resin                                       Melt Index        2.0       D-1238                                            Density           0.926     D-1505                                            Molecular Weight  89,000    --                                                % by Weight Octene-1                                                                            4.8       --                                                High Density Polyethylene Resin                                               Melt Index, g/10 min.                                                                           0.35      D-1238                                            Density, g/cc     0.963     D-1505                                            Tensile Yield               D-638                                             lbf/in.sup.2      4100                                                        kgf/cm.sup.2      288                                                         Elongation, %     800       D-638                                             Flexural Modulus            D-790                                             lbf/in.sup.2      205,000                                                     kgf/cm.sup.2      14,400                                                      Hardness, Shore D 70        D-1706                                            Izod Impact, ft                                                               lbf/in of notch   6.9       D-256                                             Tensile Impact              D-1822                                            ft lbf/in.sup.2   60                                                          cm kgf/cm.sup.2   128                                                         Brittleness Tempera-                                                          ture              <-70      D-746                                             Vicat Softening                                                               Point                       D-1525                                            ______________________________________                                    

The details and manner of producing the laminar tubular structures ofthe present invention will be apparent from the following specificexamples, it being understood, however, that they are merelyillustrative embodiments of the invention and that scope of theinvention is not restricted thereto.

In the subsequent examples the apparatus which was actually used to formthe multi-wall thermoplastic tubing corresponded essentially to thatshown in FIG. 1 of the drawing. Also, the resinous material employed inthe following examples had the physical properties as outlined inpreceding Table I.

EXAMPLE 1 (Comparative Prior Art Film)

A dual wall tubular thermoplastic film was coextruded with an innerlayer of branched low density polyethylene homopolymer and an outerlayer of a blend of crystalline high density polyethylene homopolymerwith ethylene-vinylacetate (EVA) copolymer (18% vinyl acetate byweight), and ordinary fractional melt branched low density polyethylene.The homopolymeric inner layer consists of about 96 parts by weight ofbranched low density polyethylene and 4 parts of black master batchcolorant. The outer layer consists of a melt blend mixture of about 35wt% crystalline high density polyethylene homopolymer, 35 wt% EVAcopolymer, 25 wt% branched low density polyethylene and 5 wt% of redwoodmaster batch colorant. The master batch colorants are prepared fromabout 50 wt% inorganic pigment and 50 wt% ordinary low densitypolyethylene.

The inner and outer layers are melt extruded concurrently from extruders12 and 11, respectively, forming a multilayer film having an averagethickness of about 1.5 mils. configuration as they flowed through die13. The molten tubes exit from die 13 as concentric tubes throughorifices 14 and 14' whereupon they subsequently merged together to formthe laminar tube 15 as shown in FIG. 1. The extruder processingconditions including pressures, temperatures and die orifice dimensionsemployed for this, and the following example, are set forth insubsequent Table II which also includes data on the physical propertiesof the multi-wall extruded film produced. No separation of the twolayers occurred when the resultant laminar film was repeatedly flexed.The branched low density polyethylene layer of the laminar filmconstituted approximately 78% of the overall thickness of the laminate.

EXAMPLE 2

The procedure of Example 1 was followed, however, in this case the outerlayer of the tubular film construction comprised a major amount oflinear low density polymer. The structure was further modified in thepresent example in that the outer laminar layer comprised about 75% byweight of a linear, low density ethylene-octene-1 copolymer containingabout 4.8% by weight of octene-1; 20% by weight of high densitypolyethylene and about 5% by weight of a master batch comprising 50% byweight of inorganic pigment and about 50% of weight of low densitypolyethylene as a carrier.

EXAMPLE 3

The tubular blend comprising the outer laminar layer was identical withthat defined in preceding Example 2, however, the total thickness of theouter laminar layer comprised about 26% of the overall laminatethickness.

EXAMPLE 4

The tubular laminar construction was prepared in accordance with theprocedure defined in Example 1, however, in this case the externaltubular layer comprised 22% of the overall laminate thickness.Additionally, the outer laminar layer blend in this example comprised ablend of about 60% by weight of the ethylene-octene-1 copolymer; 20% byweight of high density polyethylene; 5% by weight of the lowdensity-inorganic pigment colorant; and about 15% by weight of ordinarybranched low density polyethylene as hereinbefore defined. cl EXAMPLE 5

A tubular laminar construction was prepared in accordance with theprocedure set forth in Example 1, wherein the overall thickness of theouter laminar layer was approximately 22%. In this case, the resin blendcomprising the outer layer of the tubular laminate comprised 65% byweight of ethylene-octene-1 copolymer; 30% by weight of high densitypolyethylene and 5% by weight of the pigmented master batch material.

The physical properties of the tubular laminates prepared in accordancewith the preceding Examples are set forth in following Table 2. Table 3sets forth the process conditions which were employed to produce thelaminar structures as described in preceding Examples 1 through 5inclusive.

                  TABLE 2                                                         ______________________________________                                        Example           1      2      3    4    5                                   ______________________________________                                        Outer Layer                                                                   Percentage of Total bags                                                                        22%    22%    26%  22%  22%                                 Ethylene - - olefin (%)                                                                         --     75     75   60   65                                  HDPE (%)          35     20     20   20   30                                  Redwood Masterbatch (%)                                                                         5      5      5    5    5                                   LDPE (%)          25     --     --   15   --                                  EVA (%)           35     --     --   --   --                                  Inner Layer       78     78     74   78   78                                  LDPE              96     96     96   96   96                                  Black Masterbatch (%)                                                                           4      4      4    4    4                                   Elmendorf Tear                                                                            MD     MD     447  549  547  550  582                             (6 MS)      TD     TD     222  176  202  199  210                             1% Secant Moduls   MD     24.3 24.9 25.8 26.6 28.6                            (K PSI)            TD     31.7 30.7 29.9 33.7 35.0                            Tensile Yield      MD     1316 1418 1296 1406 1463                            (PSI)              TD     1433 1470 1414 1546 1604                            Tensile Ultimate   MD     3588 3104 3089 3328 3185                            (PSI)              TD     2146 2095 2089 2151 2232                            Tensile Toughness  MD     482  473  508  464  527                             Ft.-lb/in.sup.3    TD     736  727  702  744  783                             Tensile Elongation MD     201  227  244  208  242                             (%)                TD     576  525  555  560  574                             Directional Spencer                                                                             70     75     78   61   71                                  Opacity (Light Transmission %)                                                                  11.9   8.2    7.0  7.2  6.3                                 ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Extruder 12: (inner layer)                                                    Barrel Dia. (in.)       6"                                                    Screw RPM               49                                                    Plastic Melt Temp., °F.                                                                        396                                                   Plastic Melt Press. (psi)                                                                             4600                                                  Extruder 11: (outer)                                                          Barrel Dia. (in.)       4.5                                                   Screw RPM               41                                                    Plastic Melt Temp., °F.                                                                        500                                                   Plastic Melt Press. (psi)                                                                             5400                                                  Die 13:                                                                       Orifice Width (in.) outer                                                                             .040                                                  inner                   .040                                                  Tubular Film:                                                                 Layflat Width (in.)     72                                                    Wall Thicknesses (mils)                                                       Inner Wall              1.2 mil                                               Outer Wall              0.3 mil                                               ______________________________________                                    

As will be apparent from the foregoing Examples and Tables, it has beenfound that blend compositions comprising a linear low density copolymerof an ethylene-alpha-olefin such as octene-1 when blended together withan appropriate amount of a high density polyethylene resin providesexcellent resistance to tear and high modulus properties. Moreover, suchproperties are either equivalent or superior to prior art blend mixturessuch as those containing high density polyethylene and large amounts ofordinary low density polyethylene and/or ethylene-vinylacetate (EVA)copolymer which have been employed in prior art constructions.

Although the present invention has been described with preferredembodiments, it is to be understood that modifications and variationsmay be resorted to, without departing from the spirit and scope of thisinvention, as those skilled in the art will readily understand. Suchmodifications and variations are considered to be within the purview andscope of the appended claims.

What is claimed is:
 1. A laminar film structure comprising at least onelayer comprising a general purpose, low density polyethylene resin andlaminated thereto a second thinner layer comprising a resinous blend ofa high density polyethylene and a linear low densityethylene-alpha-olefin copolymer, wherein said blend contains a majoramount of said copolymer, said layer of general purpose, low densitypolyethylene having a thickness on the order of from about 65% up toabout 85% of the overall thickness of said laminate.
 2. A laminar filmstructure in accordance with claim 1 wherein said second layer comprisesfrom about 10 percent up to about 50 percent of the total laminar filmthickness.
 3. A laminar film structure in accordance with claim 1wherein said alpha-olefin comprises about 3 up to about 15 carbon atoms.4. A laminar film structure in accordance with claim 1 wherein theconcentration by weight of said alpha-olefin in said copolymer is fromabout 1.5 percent to about 10 percent.
 5. A laminar film structure inaccordance with claim 1 wherein said copolymer has a density below about0.94 grams per c.c.
 6. A laminar thermoplastic bag structure comprisingat least two layers, an inner layer and an outer layer, said inner layercomprising a general purpose low density polyethylene and said outerlayer comprising a resinous blend of a high density polyethylene andethylene-alpha-olefin copolymer, wherein said blend contains a majoramount of said copolymer said layer of low density polyethylene having athickness on the order of from about 65% up to about 85% of the overallthickness of said laminate.
 7. The laminar thermoplastic bag structureof claim 6 wherein said copolymer has a density below about 0.94 gramsper c.c., said alpha-olefin is present in the copolymer in aconcentration by weight of about 1.5 percent to about 10 percent, andsaid alpha-olefin comprises about 3 to 15 carbon atoms.
 8. The bagstructure of claim 6 wherein said copolymer is a linear low densitycopolymer consisting essentially of ethylene copolymerized with about 2to 10% by weight of at least one alpha-olefin comprising butene-1,hexene-1, 4-methylpentene-1 or octene-1.
 9. The bag structure of claim 6wherein said copolymer consists essentially of ethylene copolymerizedwith about 4.8% octene-1having a melt index of about 2, and a density ofabout 0.926; and wherein said high density polyethylene has a melt indexof about 0.35 and a density of about 0.963.